Large-scale solar concentrator and storage systems currently in use typically include a field of solar reflectors that focus light onto a tower-mounted receiver structure. The receiver structure may be a direct absorption type (e.g., molten salt flowing down a vertical surface) or a tube type (e.g., with water flowing through tubes). The thermal storage medium absorbs the thermal energy and, in the case of water, turns to steam or may be used to generate steam for use in a conventional steam turbine system for generating electricity. The heated medium may be transferred to an insulated storage tank so that, when the sun does not shine, hot medium from the storage tank is available to provide heat for the power cycle.
Solar power towers have several inefficiencies that adversely affect their economics. For example, because the boiler tubes in the receiver are uninsulated and exposed to the ambient environment, such systems incur high heat losses due to air convection and radiation from hot surfaces. Further, the storage medium has to be pumped up to the elevation of the boiler, which requires significant pumping power. As a result, the overall efficiencies of solar tower systems are relatively low—about 20%. Moreover, capital costs to build the solar power towers to support the heavy receiver are high.
Some of the drawbacks of tower-mounted receivers are overcome in alternative systems, where the tower supports secondary mirrors that re-direct the solar energy into a receiver on, or preferably partially buried in, the ground. On the downside, the use of secondary mirrors further reduces the efficiency of converting solar into thermal energy. The system efficiency is further constrained by the maximum solar energy density that the secondary mirrors can withstand without damage due to overheating.
An additional constraint on the ability of solar power systems to store high energy densities is posed by the temperature range in which the storage medium is chemically stable. Molten salt compositions currently in use, such as nitrate mixtures, decompose as temperatures approach 600° C., thus limiting the maximum temperature and, accordingly, the energy storage density of the system and the efficiency of power cycles downstream of the receiver.
Accordingly, there is a need for solar power systems that facilitate efficient solar-energy conversion, high-density energy storage, and robust and continuous power supply, preferably at low capital cost.